Randy,
If lower compression allows you to save money on fuel costs, at a penalty for some efficiency, cost wise, wouldn't it be cheaper to fly. "Less efficient no matter what" I can get 87 octane regular for $3.5 a gallon, a 2 dollar a gallon savings, or 17 bucks an hour at 8.5 gallons per hour. The amount of fuel you burn will probably be greater I agree, but that's a lot of money to save. And before we go into why 87 octane car gas is **** and we all shouldn't use it, this is strictly a cost/efficiency comparison.
I think you will agree, KR in his posted did not talk about the cost of fuel. He talked about efficiency which is usually far more important. Particularly where range is concerned.
I understand that the Americans reading this do not get it, but if you fly in Australia, you are damned lucky to get Avgas at most places you fly, let alone a good source for BP98 or something that is known to be good mogas. If all you do is fly from your home field and do local jollies, then cart your 20L drums and think you are saving some money. You are but not a lot. Travel anywhere, and how do you cart around 7 x 20L drums? Or do you take one and pay for 7 taxi fares in and out of town at $40 return trip.
There is more to efficiency when you start flying around the country. This is why most down here fit tip tanks or build in more in the wings. because we need to. Unlike the USA where at 10,000' you can close the throttle and glide to 1, 2, 3 maybe more airfields all with Avgas on the field. I can show you a list of sealed runway towns that have no fuel and you would be lucky to get BP98 at all. That is 94AKI or (R+M)/2 in your language. You might get 91 (our RON95).
Sid,
Simple answer is vapour lock on the IO's. But to help answer your question better, you can run ANY gasoline in your engine, it will run. However there is more to fuel chemistry than even I understand. Far more, and I have a more involved knowledge than most. Take a look at the Petersen STC list. Note they are all carb engines and ask why? There is no problem burning bog standard mogas, if your engine will run it, but the certification limits have margins for all sorts of things and doing so erodes or wipes out the margins.
The three big issues to me are these. MON (detonation margin. Now with small HP engines that run cool on cool days, they will never detonate, except if the fuel has a tiny bit of JetA or something in it. How well do you know your fuel???
Next is RVP, so the problems of vapour locking concern me a bit, in the RV10 on a hot day climb through 6-8000' without the boost pump on and see what happens. Better still do it on a hot day after the plane has been sitting in the sun. And that is with avgas. Do it with mogas with an even higher RVP
Olefins : This deserves the triple
and when you understand the serious nature of what gum content of fuel is, and compare avgas with ordinary ULP you will see why I will not run mogas?EVER. I have a almost new Yamaha 175HP V6 outboard on my ski boat. I reckon I can whip the carbs of that and clean them, refit quicker than most outboard mechanics. Don't ask me why!
If you don't fly your plane as much as you drive your car, how will a similar event to my boat go for you??.ohhhh let me convert that?.about 400' after takeoff, not sure how to convert returning from a bit of fishing in the bay, but anyway you get my point.
Now there will be hundreds on VAF that will jump up and down and argue and defend their position on mogas. I am not here to argue that they are not doing it. I know they are. Maybe many of them have never had a gum build up in a fuel servo, fuel spider or nozzle, but a blocked main jet or similar will get their attention one day.
Study the fuel spec's, get to know what is what, make up your own mind.